In the fabrication of semiconductor integrated circuits and devices, a layer of silicon nitride (Si.sub.3 N.sub.4) is often used as a passivation layer to prevent contamination of the device by air, moisture, mobile ions, and other impurities which might cause corrosion and degredation of device performance. A silicon nitride layer may also be used as a mask during such processes as ion implantation, ion diffusion, and metal deposition.
Silicon nitride layers have been prepared using a photochemical reaction, i.e. a chemical reaction which is induced or initiated by radiation. One process for depositing a Si.sub.3 N.sub.4 layer uses a photosensitized reaction between silane (SiH.sub.4) and hydrazine (N.sub.2 H.sub.4), as discussed by M. G. Collet, in the article entitled "Depositing Silicon Nitride Layers at Low Temperature Using a Photochemical Reaction," in the Journal of the Electrochemical Society: SOLID STATE SCIENCE AND TECHNOLOGY, Vol. 116, No. 1, January 1969, pages 110-111. In such a process, mercury (Hg) is excited by radiation of a particular wavelength (2537 Angstroms) to form mercury in an excited state (Hg.sup.*). The Hg.sup.* then collides with the SiH.sub.4 and the N.sub.2 H.sub.4 to cause these molecules to form radicals which then react to form Si.sub.3 N.sub.4. Further studies of this process, as well as of a fast-flow system which uses a mixture of SiH.sub.4 and ammonia (NH.sub.3), are reported by C. H. J. v. d. Brekel and P. J. Severin, in an article entitled "Control of the Deposition of Silicon Nitride Layers by 2537 A Radiation," in the Journal of the Electrochemical Society: SOLID STATE SCIENCE AND TECHNOLOGY, Vol. 119, No. 3, March 1972, pages 372-376.
In order to improve upon the films prepared in accordance with the teachings of the above references, since these films were found to have a high incidence of pin-holes, a process for the preparation of low temperature photonitride films of high quality and free of chemically bonded oxygen was discovered and set forth in copending application Ser. No. 910,704, assigned to the present assignee now U.S. Pat. No. 4,181,751 classified 427/94. By the process of the latter invention, a silane getter technique is used to remove oxygen and moisture from the nitrogen-containing reactants, in conjunction with a mercury vapor photosensitized reaction of a predetermined vapor mixture of silane, ammonia, and hydrazine at temperatures from 100.degree. C. to 300.degree. C.
In the above prior art processes, however, the photochemical vapor deposition process is initiated by the transmission of ultraviolet radiation (2537 A) through a quartz window which forms the top of the vacuum deposition chamber. During the deposition process, the quartz window also becomes coated with the deposited material (e.g., silicon nitride). This coating thus diminishes the transparency of the window, which diminishes the amount of radiation which can enter the deposition chamber to initiate the photochemical reaction, and thus impedes the rate of deposition. It is the alleviation of this problem of diminished window transparency to which the present invention is directed.
Certain vacuum systems, such as for the deposition of aluminum, have used a window assembly comprising a clear Teflon film that is manually advanced in order to permit the operator to visually monitor the deposition as it occurs in the chamber. Such a system is described in the catalog of AIRCO Temescal of Berkeley, California, entitled "Viewvac Viewing Ports Model VV400." In such a system, the window is provided for convenience and is not an integral part of the deposition reactions and process.